Measurement of extracellular tissue fluid may be of importance in a range of situations. This is particularly so in the case of lymphoedema which is a condition characterised by excess protein and oedema in the tissues as a result of reduced lymphatic transport capacity and/or reduced tissue proteolytic capacity in the presence of a normal lymphatic load. Lymphoedema normally occurs in a limb and may cause pain, scarring and reduced limb function. The condition is incurable, progressive, often disfiguring and physically disabling. Its course, however, can be arrested or slowed by intervention using physical therapy, compression bandaging, massage and other physical techniques.
Acquired or secondary lymphoedema is caused by damaged or blocked lymphatic vessels. The commonest inciting events are surgery and/or radiotherapy. However, onset of lymphoedema is unpredictable and may develop within days of its cause or at any time during a period of many years after that cause.
There is a need for an accurate and effective technique to detect the onset of lymphoedema, assess its severity and monitor its response to treatment. The simplest known technique involves measurement of limb circumferences and comparison with a paired unaffected limb. A further technique is available by way of immersion of the affected part and measurement of displaced liquid with subsequent comparison against the result of the same measurement performed on an unaffected limb.
It is also known to use multiple frequency bioelectrical impedance analysis (MFBIA) to assess lymphoedema (Watanabe et al., 1989, Lymphology 22:85). The authors noted that when a low frequency voltage is applied to tissue, the impedance of the cell membrane is substantial. With increased frequency, the impedance of the cell membrane decreases and current is able to flow through both extracellular and intracellular fluids. The results obtained by Watanabe et al were subject to analysis of equivalent resistivity of extracellular and intracellular fluid calculated after measurement of electrical bioimpedance at multiple frequencies. Further development of the technique was subsequently disclosed (Ward et al., 1992, European Journal of Clinical Investigation 22:751) in which MFBIA was used and the impedance at zero frequency was estimated by extrapolation. Differences were then calculated between left-hand and right-hand sides of patients for the impedance calculations 50 kHz and 0 kHz frequency. The bilateral all differences in impedance between a group of controls and a group of affected patients were significant. This test relies on the use of a multifrequency bioimpedance meter and relatively complex analysis of the results to provide an indication of lymphoedema.
U.S. Pat. No. 5,372,141 describes a body composition analyser that provides information in relation to body fat and ideal body weight. The analyser compares the bioimpedance of the body “network” against a reference network of known impedance. It is, however, of little or no use in assessing tissue oedema.
U.S. Pat. No. 4,947,862 discloses an analyser to determine the amount of body fat on a patient. The analyser uses a high frequency low-voltage signal in the body and measures magnitudes and phase shift of the induced signal but again is of little use in measuring tissue oedema.
It would be of advantage to provide a method for determining the presence of oedema and, in particular, lymphoedema by measurements taken at a single frequency. It would further be advantageous to produce a device for measuring bioelectrical impedance at a single frequency and analysing that measurement to produce an indicator of the presence of oedema.